Photoresponse of new azo pyridine functionalized poly(2-hydroxyethyl methacrylate-co-methyl methacrylate)

A new azo polymer containing photoisomerizable azo pyridine functionalities was synthesized via Mitsunobu reaction of 4-(4-hydroxyphenylazo)pyridine with poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) (p(HEMA-co-MMA)) for creating new photochromic materials. The resulting polymer with azo pyridine side groups was characterized for structural, thermal, and optical properties. UV–vis, 1H NMR and IR spectroscopies confirmed that all hydroxyl groups in p(HEMA-co-MMA) were substituted with azo dye. The obtained azo copolymer exhibited high thermal stability (around 240 °C) and a glass transition temperature (113 °C), promising for applications. The trans-to-cis isomerization upon UV irradiation and the thermal back reaction of the azo chromophore in the copolymer in the solid state was studied. A photostationary state with 50% content of cis-isomers upon 6 min of UV irradiation was reached, and during 48 h dark relaxation at ambient temperature, all cis-isomers converted to the trans form. Additionally, the possibility of efficient photogeneration of surface relief gratings with high amplitude of azo copolymer surface modulation was demonstrated.


Polymer film preparation
The homogenous solutions of polymers in NMP were filtered through 0.45 mm membranes and cast onto clean glass substrates (2×2cm).The residual solvent was removed from films by heating them for 24 h at 85 °C in vaccum, near the polymer Tg, that is the temperature AT which the polymer segmental motion is large enough to free possibly trapped solvent molecules.
In the surface relief grating recording experiment, the thickness of the prepared polymer layer was 2.7 m.It was determined using a Dektak XT stylus profiler (by scanning the surface of the polymer layer with an intentionally made scratch).

Polymer characterization
1 H NMR spectroscopy experiments of the investigated materials were carried out on an Avance II UltraShield Q3 Plus Bruker MT 600 MHz spectrometer (Germany).Tetramethylsilane (TMS) and deuterated dimethyl sulfoxide (DMSO-d6) were used as an internal standard and as a solvent, respectively.
FTIR-ATR spectroscopy data were received by using a Nicolet 6700 FTIR apparatus (Thermo Scientific, MA, USA).The FTIR spectra were acquired in the range of 4000 -500 cm -1 , at the resolution of 0.09 cm -1 for 64 accumulated scans.Wide-angle X-ray diffraction (WAXD) analysis was performed using an HZG-4 diffractometer (Carl Zeiss, Jena, Germany).The samples were examined working on Bragg geometry of 2θ from 5 to 50° using Ni-filtered Cu-Kα radiation with a wavelength λ = 1.54051Å.
Differential Scanning Calorimetry (DSC) data were recorded on a TA-DSC 2010 apparatus (TA Instruments, Newcastle, DE, USA).Before the analysis, the polymer samples were vacuum dried at 60 °C for 24 h.The samples were analyzed with a heating rate of 20 °C min -1 .The glass transition temperature (Tg) was taken as a midpoint of the heat capacity step change registered in a second run.
Thermogravimetric analysis (TGA) was done with a TA TGA55 apparatus with a heating rate of 15 °C•min -1 in a constant stream of nitrogen (25 ml•min -1 ) and a temperature range from 25 °C to 800 °C.The compounds before measurement were dried in an oven at 100 °C or 80 °C for 24 h in a glass vessel.
UV-Vis spectroscopy.The measurements of UV-Vis absorption spectra were measured for polymer solutions in NMP (c=10 -5 mol/l) and films casting on a glass substrate using a Jasco V-750 (Jasco Inc.).The cell path used for the spectroscopic measurement of the solution was 10 mm.
Atomic Force Microscopy.The thickness of the film was determined by atomic force microscopy (AFM) using a TopoMetrix Explorer device, operating in contact mode, in air, and constant force regime.

Loading level of 4-(4-hydroxyphenylazo)pyridine
The chromophore loading level of the azo polyimides was determined by the UV-Vis spectroscopy using the Lambert-Beer Law

Fig. S2 .
Fig. S2.UV-Vis spectra of azo(HEMA-co-MMA) in NMP solution and in film after 6 minutes of UV-light irradiation.
Molecular weights (Mn, SEC) and dispersity indices (Ð) were determined by size-exclusion chromatograph (SEC, Ultimate 3000) equipped with an isocratic pump, autosampler, degasser, thermostatic box for columns and differential refractometer RefractoMax 521 Detector and DAWN®, Multi-Angle Laser Light Scattering (MALLS) detector.ASTRA data analysis software was used for data collecting and processing.The RID calculated molecular weight was based on calibration using linear polystyrene standards (Mp = 580 -3000 000 g/mol).Precolumn guard 5μm 50 × 7.5 mm and double PLGel 5μm MIXED-C and MIXED-D 300 × 7.5 mm column were used for separation.The measurements were carried out in THF (HPLC grade) as the solvent at 35 °C with flow rate of 1 mL/min.
[6].The degree of the chromophore substitution in the polymer was calculated from the calibration curves obtained from the UV-Vis spectra of the chromophore solution with various concentrations (1×10 -6 -1×10 -4 mol/l) for the 4-(4hydroxyphenylazo)pyridine (equation of the calibration curve was y = 9.2756x -0.449).From the comparison of the peak absorbance of the polymer in solution with the known concentration and the calibration curve, the degree of the functionalization was estimated.The absorbance of azo polymer (c = 10 -5 mol/l) was 0.1663 for azo(HEMA-co-MMA).